As the blizzard roared through the Northeast, Airlines canceled flights by the thousands and air travel ground to a virtual standstill.

More than 6,600 flights have been canceled since Thursday in the United States, with most of those coming at airports affected by the storm.

Across the region, flights were expected to be back on close to normal schedules on Sunday, though flight-tracking website FlightAware said Sunday cancellations stand at 326 as of 12:00 a.m.

Even with flights resuming, the airlines recommended that travelers confirm departure before heading out to the airport.

As for the cancellations already tallied, more than half of the those were announced even before the first flakes fell in the Northeast. That highlights a trend in recent years in which airlines have proactively canceled flights ahead of a big storm instead of trying to operate right up until the worst conditions arrive.

Cumulatively, airlines axed about 4,136 flights across North America Friday, and another 2,476 Saturday, according to flight tracking service FlightStats.

More than half of those cancellations have come at just four airports: Boston as well as the three big New York City-area airports of Newark, LaGuardia and JFK, according to FlightAware.

With such a large number of cancellations, the flight disruptions in New York and the Northeast are all but certain to ripple through airports across the nation. A flight from Houston to Los Angeles, for example, could become delayed or canceled if the aircraft or crew scheduled to fly it gets knocked off schedule because of problems in Boston or New York.

Perhaps as evidence of that, FlightAware is reporting nearly 100 preemptive Friday and Saturday cancellations for Houston Bush Continental. At Miami, nearly 90 Friday and Saturday flights had been scrapped by Thursday evening. Both airports are hubs for airlines that also have hubs in the Northeast, meaning many of the cancellations in Houston and Miami are likely part of the ripple effect of the cancellations in Newark, JFK, Boston and elsewhere.

Against that ominous air-travel prognosis, airlines have relaxed rebooking rules and are encouraging customers to change their planes to avoid the storm's chaos.

"Delta is closely monitoring conditions along the storm's forecast path and encourages customers to consider moving up, postponing or re-routing their travel to avoid possible inconvenience from expected flight delays," the airline said in announcing its winter-weather policy.

Delta's waived-fee policy mirrors that of the other big airlines. American, Delta, United, US Airways and most other big carriers charge $150 or more for changing tickets - a fee that comes in addition to any difference in fare that might result.

Low-cost carrier Southwest - which flies more domestic passengers than any other U.S. carrier - does not charge a change fee, but said Wednesday that customers at several would be able to make a change with no recalculation of their fares.

JetBlue, one of the busiest airlines in the Northeast, is waiving fees at a dozen airports because of the storm. JetBlue operates its two biggest hubs at New York JFK and Boston, airports that are both in the storm's path.